In the long term evolution (LTE), to realize a further increase of the data rate, a short transmission time interval (TTI) such as, for example, 1 [ms] is employed. Reduction of the TTI reduces a round trip time (RTT) necessary for controlling retransmission and, thereby, any delay in a system is reduced.
On the other hand, in a system employing the short TTI, for example, for a mobile station (user equipment: UE) located on an edge of a cell, sufficient energy cannot be secured for transmission because the transmission electric power is limited. In this case, a state occurs where a base station cannot accurately receive data from the mobile station due to the radio wave quality. In this case, it is important how significantly the delay caused by the data retransmission is reduced.
A technique is disclosed as a retransmission method for data in the LTE system, according to which data including an error and retransmitted data are coupled, thereby, a correction capacity is improved, and the retransmission frequency is suppressed, based on hybrid automatic repeat request (HARQ) control (see, e.g., Published Japanese-Translation of PCT Application, Publication No. 2010-530709).
However, according to the HARQ control, a delay corresponding to eight TTIs [s] is generated judging from the transmission time interval (TTI) from the time when the data is transmitted through the time when a receiving apparatus receives the data to the time of a response thereto (an ACK or NACK response). When the retransmission is repeated for n times, a delay corresponding to n×RTT (eight TTIs) is generated and this becomes an obstacle against realization of a high data transmission speed.
In the LTE, a retransmission procedure using TTI bundling is employed as an approach against the delay (see, e.g., Japanese Laid-Open Patent Publication No. 2009-253981). The “TTI bundling” is an approach to facilitate improvement of the reception quality by consecutively transmitting packets each including same data for several TTIs and coupling the data with each other in the reception side.
The “TTI bundling” is a method of improving the reception quality without executing any retransmission procedure in response to a reception response by transmitting the same data consecutively for several TTIs from the first one and coupling the data received for several TTIs in the reception side. Under the 3GPP, it is defined that a TTI bundle size=4; in the first transmission session, new data×1 and retransmission data×3 (that is, basically, four data whose contents are the same as that of the new data) are consecutively transmitted in four TTIs without waiting for any reception response; and the reception side can receive one normal data acquired by coupling the data for four TTIs consecutively received at the reception side. When the retransmission frequency is high, improvement of the data transmission efficiency and realization of reduced delays are enabled by switching the scheme from a non-bundling HARQ scheme to the TTI bundling scheme.
However, according to the conventional techniques, a problem arises in that the communication efficiency cannot be sufficiently improved.
According to the existing TTI bundling, only one data can be transmitted in four TTIs. The TTI bundling operates when the radio wave state is not so favorable, and is effective. However, the redundant resources are useless in contrast when the radio wave state is gradually improved and among the four TTIs, for example, for two TTIs that is a half thereof or more, the reception quality (for example, acquired by a cyclic redundancy check (CRC)) becomes high for the data thereof. In this case, the switching of the scheme from the TTI bundling scheme to the HARQ scheme may be considered. However, the transmission efficiency does not vary with either one of the TTI bundling scheme and the non-bundling scheme although the CRC is improved for only two TTIs.
In addition, to switch the scheme from the TTI bundling scheme to the non-bundling HARQ scheme, resource reallocation, etc., are necessary based on an RACH sequence (mutual communication between the base station and the mobile station triggered by an RRC connection reconfiguration from the base station to the mobile station). Therefore, the communication between the base station and the mobile station is increased and more time is necessary. Consequently, when the TTI bundling scheme is used, provision of a control method is effective that enables the transmission efficiency to be variable depending on the radio wave state, maintaining the use of the TTI bundling scheme.